Thermal Shock in Hand-Fired

Modern and vintage hand fired coal stove are similar to a wood stove and in some cases can burn either. They need to be regulated and fed by hand usually every 12 to 24 hours depending on your usage. They require no power to operate making them ideal for rural settings with long power outages.
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JB Sparks
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Stove/Furnace Make: Harman - Chubby
Stove/Furnace Model: Harman: SF160 - Chubby Sr.
Location: north central Mass.

Post Thu. Dec. 11, 2008 9:09 am

I have a SF160 boiler hooked up in parallel to my oil boiler with a circulator between them running 24/7. The four heating zones work normally as they would when the oil boiler is in use. When any zone calls for heat and that zones circulator comes on the boiler water temp. will drop about ten degrees and then recover in about 15 to 30 minutes. However when the super-saver DHW circulator comes on to heat the DHW the boiler water temp. will drop about 30* and take up to an hour to recover. So my question, is that a thermal shock to my boilers ( the 30* drop in boiler water temp.)and if it is has anyone got a fix? And even if it is not considered a thermal shock anyone have an idea how to prevent the 30* drop in boiler temp?

Thanks
JB
Last edited by JB Sparks on Thu. Dec. 11, 2008 4:00 pm, edited 1 time in total.

packard bill
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Hand Fed Coal Boiler: DS Machine and homebuilt
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Location: Pennsylvania,USA

Post Thu. Dec. 11, 2008 9:59 am

Hi JB.Not knowing what water temps (boiler vs dhw tank) you operate at, it would be hard to ascertain "shock". If boiler temp is, say, 195 deg. and you're introducing 140 deg. water from dhw, you may be realizing some shock.
100 feet of 3/4" pipe (including baseboard "fin") contains about 6 gals of water. That's not alot of water to reheat. Your DHW tank, on the other hand, is another story. Lots more water to bring up to temp.

You can add a mixing line (boiler water) to the intake side of the DHW circulator pump so you're not introducing water into the boiler at such a great differential, however this will slow your DHW recovery. You can set your differential tighter on the circulator control, or even increase the temp of the DHW tank water (assuming you have a tempering valve installed for the DHW). That's my 2 cents worth. Look forward to many more good ideas here.

Good luck with it.
packard bill

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Freddy
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Coal Size/Type: Pea size, Superior, deep mined
Location: Orrington, Maine

Post Thu. Dec. 11, 2008 11:24 am

Thermal shock doesn't bother a steel boiler as it does a cast iron boiler, but it's still best avoided. A by pass loop would help... that is, a 3/4 inch pipe from supply directly to the return with a ball valve in it. Partially close the valve and it will keep the boiler up to temp while it delivers water to whatever calls.
Orrington, Maine
Fred

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JB Sparks
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Stove/Furnace Make: Harman - Chubby
Stove/Furnace Model: Harman: SF160 - Chubby Sr.
Location: north central Mass.

Post Thu. Dec. 11, 2008 12:06 pm

Hello Packard bill and Freddy,

What an ingenious idea! You both are saying pretty much the same thing, let me repeat in my words so that i'm sure I got it.
My circulators are on the supply side of the loop. The house heating loops are 3/4" pipe, and the DHW is 1" pipe ( right, a lot more flow). the Dhw does not have a tempering valve, it is controled by its own Aquastat. I keep the DHW water at 130*, which means it will call for heat at a 120*. I try to maintain Boiler water between 170 to 180 degrees. So if I put in a bypass in the supply line between the circ. and the tank and run it to the boiler return line with a ball valve I can adjust the valve to control the water temp back to the boiler. That is so simply I can believe I didn,t think of that ( being a simply minded person that I am). What this probably will do is take a longer time for the DHW to come up to temp, but who cares when it would drop the water temp to a 140* it would run a long time anyway.

Thank you very much
JB

PS: Freddy, I read alot of your post. You should have your own show, you are one funny guy. I was rolling on the floor this morning after reading your answer to the stud finder question. :lol: :lol: :lol:

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JB Sparks
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Stove/Furnace Make: Harman - Chubby
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Location: north central Mass.

Post Thu. Dec. 11, 2008 12:18 pm

Ok, just check how and where to put in the bypass. another question, should I use 1" pipe as the existing pipe is or would 3/4 " pipe be ok? also would I need to put in another valve say in the return line so as to balance the flow rate between the bypass and the regular circuit?

Thanks again
JB

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SMITTY
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Coal Size/Type: Rice / Blaschak anthracite
Other Heating: Oil fired Burnham boiler
Location: West-Central Mass

Post Thu. Dec. 11, 2008 12:55 pm

Just to make you feel better about the shock factor.............

One time I came home from work on a warmer day & checked my boiler temp to see how hot it was, knowing that there was probably no call for any heat in any zone ....and it's connected to my 2 massive coils in the stove.

Temp was up to 263*!! :eek2: I turned on 2 zones at the same time to bring the temp down some. So I figure I had 50* - 60* water coming into my boiler (upstairs zone gets real cold from cold air that comes thru walls & windows).

So I figure, if my boiler didn't crack in two pieces from that shock, then I think you'll be ok. ;)
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JB Sparks
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Post Thu. Dec. 11, 2008 3:58 pm

Smitty, That is a thermal shock!!! :shock: It also shows us how well built these stoves are.

What I did for a dump zone was to hook up a little 24 volt relay that has 4 poles, one contact for each zone. When the Johnson dump relay kicks in on overfire it kicks in my relay and pulls in all four zones at the same time. That lowers the water temp. in about 2 minutes, works great.

How far west are you in this great state of Taxachusetts?

JB

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coaledsweat
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Post Thu. Dec. 11, 2008 8:16 pm

JB Sparks wrote:So my question, is that a thermal shock to my boilers (the 30* drop in boiler water temp.)and if it is has anyone got a fix? And even if it is not considered a thermal shock anyone have an idea how to prevent the 30* drop in boiler temp?


Thermal shock is not an issue with a residential boiler at the temperatures we run them at. It is most often seen in steam boilers where feed water can be 200 or more degrees lower in temperature than the receiving vessel. Repeated high temperature shocks crystallize the grain structure of the steel over time and degrade it to the point where it can fracture from the internal pressure. This takes place at much higher temperatures and pressures than we see in our home heating systems.

Your problem is recovery (30* is not a big deal). I would bump the aquastat up 10-20* on the high and low limits. Because you DHW is zoned, you are safe from a scalding situation. Do you have a valve on the loop to your DHW that you can trim the flow with? Reducing the flow will reduce the impact on the boiler.
It sounds like you have a big DHW tank, that is a lot of heat it is eating.
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JB Sparks
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Location: north central Mass.

Post Thu. Dec. 11, 2008 9:39 pm

Coaledsweat,
Thanks for the info on the thermal shock. i'm glad thats not an issue. Yes I have a ball valve on the feed side and the return side. I tried closing the return side about half way and it did'nt do anything, still drop the temp 30*. The tank is a 45 gal. superstor. the circulation pipes are 1" so it pumps alot of water. Freddy told me to put in a bypass between the supply and return with a valve to adjust. what do you think about that? it sounds like a real good idea to me.

Thanks again
JB

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Freddy
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Location: Orrington, Maine

Post Thu. Dec. 11, 2008 10:12 pm

Typically a bypass is 3/4". If you happen to have 1" I'm sure it's be OK. Just the ball valve in that pipe should do it. I've never seen the return throttled back as a temperature control.

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coaledsweat
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Post Fri. Dec. 12, 2008 9:12 am

JB Sparks wrote:Freddy told me to put in a bypass between the supply and return with a valve to adjust. what do you think about that? it sounds like a real good idea to me.


That should help, but the net heat loss just gets spread out over a longer time frame. Did you try running the boiler at a higher setting? What is it at now? A 20* rise in boiler temp may equate to only a 15-20* drop when your DHW calls for heat.

Also, I would look at trying to get your thermostat on the DHW tightened up so it doesn't wait so long before it calls for heat. If it is calling for heat on a 20* drop set it to 10*, that may cut the call time by as much as 2/3 and solve your problem without replumbing everything.
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JB Sparks
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Stove/Furnace Make: Harman - Chubby
Stove/Furnace Model: Harman: SF160 - Chubby Sr.
Location: north central Mass.

Post Sat. Dec. 13, 2008 6:43 am

Thanks guys, all good ideas.

Coaledsweat, the tank is controled by its own aquastat with a fixed 10* differential and I run the boiler at 180*, when it is up to that temp it only drops about 20* which is not bad. Cutting in a bypass is a piece of cake,i'll get to it pretty quick, but for today i'm going to enjoy having the electrical power back on. Power went out about 10 PM thursday just came back on, what a relief.

Thanks Freddy, might just as well put in a 1" bypass.

JB

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